The present invention relates to a method and to a machine for softening and stretching laminar products, particularly tanned industrial hides, in a substantially continuous manner. In this specific field, the process according to the invention is commonly termed perching and the machine for performing it is termed percher or perching knife.
Perching is performed on the hides after the tanning and partial drying treatments, after which the fibers are entangled and shortened, making the hides rigid and hard. By means of perching, the fibers are stretched, separated and aligned so as to provide the hides with their original elasticity and softness.
Heretofore, perching was performed by manually stretching the hides and rubbing them vigorously against typical knifeshaped tools. In modern automatic machines, studied to reduce or eliminate physical effort and make the treatment faster, the old manual tools have been replaced with levers, rollers or, more recently, with oscillating plates. These more recent machines offer the advantages of making the treatment continuous, considerably increasing the productivity of the treatment itself.
All the automatic machines mentioned above use a method which consists in applying, on the opposite sides of the hides, opposite forces which are substantially perpendicular to their surfaces. Said forces are applied in mutually spaced regions on one side of the hides and in intermediate regions between said spaced regions on the other side, so as to exert, between the working regions, traction forces exceeding the elasticity limit of the fibers, with consequent multi-directional elongations.
The Italian paten IT-A-883912 discloses a perching machine applying the above described method, comprising a pair of facing plates between which the hides to be treated are caused to advance; at least one of the plates moves with a reciprocating motion closer and further away from the other plate to engage the interposed hides. On at least one of the facing plates there is provided a series of mutually equispaced protruding elements which are substantially formed by pins or dowels with a rounded end which are adapted to cooperate with a series of opposite elements provided on the other plate. In this known machine, the elements of the second plate consist of cavities which are complementarily shaped with respect to the protruding elements. In an alternate embodiment, the elements of the second plate may consist of a second series of protruding elements which are arranged in the free spaces between the first elements. The elements of the plates are not directly acting on the hides to be treated, but by means of a pair of adjacent elastic belts which, by virtue of their concurrent movement, cause the advancement of the hides in a direction which is perpendicular to the movement of the plates.
Machines of this type allow considerable productivity, mainly in view of the high frequency of the transverse oscillating motion of the plates, which is generally comprised between 5 and 10 strokes per second and is coordinated with the longitudinal advancement movement of the belts. However, these machines have some known problems. In order that the machine be adapted to the different thickness and texture of the hides, the minimum distance between the plates is accordingly varied so that a variation in the degree of penetration of the opposite elements and therefore of the obtained stretching corresponds to this distance variation. It is evidently a main object to increase as much as possible the elongation of the fibers to widen the footage of the hides. Unfortunately, an excessive degree of penetration can cause indelible markings and even breakage of the hides and/or of the belts at the coupling locations of the elements.
In order to improve the degree of uniformity of the stretching for an equal degree of penetration of the plates, it is furthermore necessary to reduce the advancement speed of the belts so as to expose a same region to a repeated treatment, with the consequence, however, of reducing the machine's productivity.
Another disadvantage which is frequently observed in said known machines is the so-called boundary effect, i.e. the lack of stretching along the peripheral region of the plates. Whereas in the central regions of the plates the hides are stably locked by the active surfaces of the adjacent elements, proximate to the edges this locking action is in fact partially missing and is reducing the stretching action. A system for reducing this disadvantage consists in moving the plates closer, increasing the degree of penetration of the active elements on the entire work area so as to increase the locking forces proximate to the edges as well. However, this solution can cause such an increase in the stretching of the hides toward the center of the plates so as to cause unrecoverable damage due to the above mentioned reasons.